INTRACELLULAR CALCIUM REGULATES AMP-ACTIVATED PROTEIN KINASE ACTIVITY IN AN OSCILLATION-DEPENDENT MANNER Sungkwon Park, Eric M. England, Haibo Zhu, Jason M. Scheffler, Steve C. Kasten, Tracy L. Scheffler, and * David E. Gerrard Department of Animal and Poultry Sciences, Virginia Tech, Blacksburg, VA, 24061, USA *Corresponding author (phone: +1-540-231-9157; fax: +1-540-231-3010; e-mail:
[email protected]) Abstract—Skeletal muscle calcium signaling is important for muscle contraction, as well as regulates many cellular processes. Calcium-regulated calmodulin dependent kinase kinase (CaMKK) has recently been identified as upstream regulator of AMP-activated protein kinase (AMPK), which is energy regulator in skeletal muscle. Although there is evidence that cytosolic calcium regulates AMPK through a series of pathways, the molecular mechanisms by which calcium regulates AMPK are poorly understood. The objective of this study is to understand the function of calcium oscillations on AMPK activity and define the specific calcium-regulated signaling molecules in this pathway. AMPK activity was increased by 2 folds in muscles from mice treated with AICAR (known AMPK activator). Administration of caffeine (calcium releasing agent) for 10 d decreased AICAR-induced AMPK activity to control level. This repressed AMPK activity was blocked by dantrolene, a ryanodine receptor stabilizer. Different calcium frequencies were simulated in C2C12 myotubes by alternating media containing caffeine and dantrolene. Changes in intracellular calcium levels were confirmed by fluorescent calcium indicator, Fura2. To define the function of calcium signaling, CaMKK or CaMK was knocked down. Low frequency calcium stimulations had a positive effect on AICAR-induced AMPK activity, whereas continuous high calcium level decreases AMPK activity suggesting a biphasic control of AMPK activity by calcium.